Growing Health Importance of Dietary Flavonoids

Enter the Diverse World of Flavonoids

Flavonoids, or polyphenols, are a class of compounds present in a wide array of vegetables, fruits, beverages such as tea and red wine, and some members are found in beans and whole grains. The health effects of flavonoids have been intensively investigated over the last 10 years, and nutrition scientists urge us to consume more of them.

It has been proposed that flavonoids help protect against damage in blood vessels, thus decreasing the risk of cardiovascular diseases. In addition, they may have a role in cancer prevention, in enhancement of the immune system, and even in eye health. While they act as antioxidants, many flavonoids work in diverse ways to influence cellular functions. Actually, flavonoids represent thousands of compounds that have been categorized into subclasses-often with funny-sounding, complex names. But our goal is simple: Eat a wide variety of foods that are rich in flavonoid family members:

• Flavonols. Good sources of flavonols in the diet are onion, kale, broccoli, lettuce (the greener the leaf, the higher the content), tomato, apple, grape, berries, tea and red wine. Examples include quercetin, myricetin and kaempferol.
• Flavones like luteolin and apigenin are found in foods like celery and parsley.
• Flavonones. Abundant in citrus fruits and juices - members include hesperetin & naringenin.
• Anthocyanins are the red to purple pigments that color fruits and veggies such as red cabbage, plum, cranberry, bilberry and blueberry. Cyanidin, delphinidin and malvidin are examples.
• Catechins (proanthocyanides). Best known as green tea components, they are also found in red wine, chocolate, apples and berries. Members include epicatechin, gallocatechin and EGCG.
• Isoflavones are a special sub-class that includes lignans from flaxseeds, legumes, whole grains, fruits and vegetables, and isoflavones (genistein, daidzein) from soy foods. These compounds can mimic some of the effects of estrogens.

Past issues of Staying Healthy newsletter have reported research findings on flavonoids such as green tea catechins and brain function (April, 2006) or the anti-aging effects of blueberry anthocyanins (February, 2002). Recent studies also illustrate how the potential health effects of flavonoids are as diverse as the compounds that make up this healthful class of nutrients.

Flavonoids, Lignans and Breast Health

Results of the Long Island Breast Cancer Study project suggest that higher consumption of some flavonoids may decrease the risk of this disease (1). Comparing data from 1,434 breast cancer patients with 1,440 women without the disease revealed that the risk for developing breast cancer was reduced for those with the highest vs. the lowest total flavonoid intake. Risk was about 45% lower among post-menopausal women, though no benefit was seen for younger women. Specific flavonoids - including flavones, flavonols and lignans - were associated with risk reductions ranging from 26 to 39%, while the flavanones, soy isoflavones and anthocyanins, showed no relationship.

"These results are consistent with other studies conducted among Mediterranean women," according to the lead investigator. "Few epidemiologic studies have examined whether there is a relationship between breast cancer and dietary flavonoids. Our study proposes that dietary flavonoids can help American post-menopausal women reduce their risk of breast cancer, though further research is needed."

Quercetin and Eye Health

Evidence suggests that chronic oxidative stress may damage retinal pigment epithelial (RPE) cells, making them more susceptible to developing age-related macular degeneration. In a recent study, all 6 classes of flavonoids were tested to see if they could protect cultured human RPE cells (2). Quercetin, fisetin, luteolin and EGCG were especially effective. These promising results now need to be confirmed in clinical trials.

References

1. Fink BN, et al. Dietary Flavonoid Intake and Breast Cancer Risk among women on Long Island. Am J of Epi 165:514-23, 2007.
2. Hanneken A, et al. Flavonoids protect human retinal pigment epithelial cells from oxidative stress-induced death. Inv Ophthalmol Vis Sci. 47:3164-77, 2006.